1. Field of the Invention
This invention relates in general to hearing testing of a human being. In particular, the invention relates to recording of distortion product emissions (DPEs) of human ears. Still more particularly, the invention relates to apparatus and methods for recording DPE audiograms and input/output functions and to the minimization of random noise in the presence of DPE.
2. Description of the Prior Art
Otoacoustic Emissions (OAEs), first described in 1978, represent acoustic energy presumed to be generated by stimulus-induced, motile activity of the outer hair cells of the Organ of Corti in the Cochlea of the inner ear of a human being and other mammals. It is believed that mechanical feedback of such outer hair cells into basilar membrane motion and their related cochlear-efferent endings are part of a biomechanical gain system that is responsible for the sharp tuning and high sensitivity associated with normal hearing.
Otoacoustic emissions (OAEs) may be classified generally as spontaneous emissions and "evoked" or stimulated emissions. Stimulated emissions can be further separated into three subclasses consisting of transiently evoked emissions (TEEs), stimulus-frequency emissions (SFEs), and distortion-product emissions (DPEs). Each type of stimulus [i.e., clicks (TEEs) or low-level, continuous pure tones (SFEs) or continuous, simultaneously applied, two-tone stimuli (DPEs)] generates evoked emissions. TEEs and SFEs have an appreciable latent or delayed time period with respect to stimulus onset. DPEs, have a nonlatent or instantaneous onset. Based on the response-latency distinction, it is believed that separate subcellular components of the outer hair cell support the generation of delayed versus instantaneous evoked OAEs. For example, the stimulus-induced movements of the stereocilia bundle likely generate the nonlatent DPEs, while the motile activity of the lateral regions of the hair-cell membrane likely produces the latent TEEs and SFEs.
D. T. Kemp proposed that a transient OAE could be a diagnostic tool in the examination of impaired hearing. Kemp, Stimulated Acoustic Emissions from within the Human Auditory System, J. Acoust. Soc. Am., Vol 64, No. 5, pp 1386-1391, November 1978. After Kemp's discoveries became known to the art of hearing research, a number of researchers investigated the status of stimulated OAEs in people with normal hearing and with hearing impairments.
Early studies established that emissions are present in essentially all normally hearing individuals and that such emissions are reduced or eliminated in regions of sensorineural hearing loss. Moreover, it became apparent that, of the three types of stimulated emissions, SFEs could not be simply applied in practical settings, because they require the utilization of complex methods of analysis in order to separate them from the eliciting stimulus.
Of the remaining evoked-emission types, that is TEEs and DPEs, TEEs have received, by far, the most attention as potential clinical measures of cochlear function. The development of transiently evoked otoacoustic emissions (TEEs) has, in fact, reached an advanced level in that a computer-based commercial device is currently available to the audiologist: D. T. Kemp, et al., Acoustic Emission Cochleography--Practical Aspects, Scand Audial Suppl. 25, pp. 71-95, 1986; Peter Bray and David Kemp, An Advanced Cochlear echo Technique Suitable for Infant Screening, British Journal of Audiology, 1987, No. 21, pp. 191-204.
The form of a TEE from a given ear is subject to the invariable influence of fixed-frequency emissions that are unique to that ear. In the presence of idiosyncratic frequencies, including the spontaneous and stimulus-frequency otoacoustic emissions, as well as the TEEs themselves, cochlear function, at specific frequencies (e.g., audiometric-test frequencies), cannot be uniquely assessed. Consequently, TEE testing appears most useful as a screening device for estimating the absence or presence of reasonably normal hearing.
In contrast to the significant attention that TEEs have received as potential clinical indicators of outer hair-cell cochlea function, DPEs have not been extensively investigated as the basis of an objective test of hearing impairment.
3. Identification of Objects of the Invention
In view of the inherent problems of using TEEs and SFEs for hearing testing, it is a general object of the invention to provide method and apparatus for using DPEs as the basis of an objective hearing test, both for normal and hearing-impaired ears.
It is another object of this invention to provide a method and apparatus by which an ear may be tested, using DPEs, for hearing capability at any frequency between approximately one and eight kHz. In other words, it is an object of this invention to provide a method and apparatus for creating a DPE audiogram for a human being.
It is another object of this invention to provide a method and apparatus by which an objective measure of the hearing capability at a particular frequency varies as a function of stimulus level, so as to permit a complete evaluation of cochlear function at both threshold and suprathreshold levels of stimulation. In other words, it is an object of the invention to provide a method and apparatus for creating a DPE input/output function for a human being.
It is another object of this invention to provide a method and apparatus by which DPEs may be used to evaluate remaining outer hair-cell function in hearing impared human beings having a hearing loss up to 45-55 dB HL.
It is another object of this invention to provide noise reduction method and apparatus for reducing the noise signal which contaminates the measurement of the DPE, thereby providing a system which may be used in a noisy environment such as a doctor's clinic or other hearing screening facility.